Various methods have been employed for determining the porosity of petroleum-bearing reservoir rock. Such porosity measurements are used quantitatively in characterizing the reservoir rock for the purpose of determining hydrocarbon productivity and calculating reserves. One long-standing method is the direct analysis of cylindrical core samples that are taken during the drilling operation. Methods of analysis based on core samples have the advantage of being able to provide detailed and very accurate data of the reservoir quality at precisely known depths. The principal disadvantages of relying on core samples is that collecting the samples is both time-consuming and expensive, as is the processing of the core slabs to prepare samples for the one or more eventual analytical processes from which the data can be developed.
Down-hole "electric" or petrophysical logs are the most common means of assessing reservoir quality. The advantages of this technique are that the data is available immediately after the drilling of the well and the data can be obtained over the entire portion of the "open" well-bore. The disadvantages of this technique are that the data is not available until after the well is drilled, and this information cannot be used to assist in making drilling decisions. Measurement While Drilling ("MWD") or Logging While Drilling ("LWD") techniques partially overcome this deficiency; however, the cost for this service is very high and not all petrophysical tools can be utilized.
Another method for evaluating reservoir rock is based on the pyrolysis of rock cuttings that are carried to the surface during drilling operations by the drilling fluid, or "mud." Collection of rock cuttings associated with known depths is a well established procedure in petroleum drilling operations. Depth assignment to the cuttings is based on calculations which take into account drilling fluid circulation rate, hole geometry, fluid viscosity and weight, and other parameters. Collecting cuttings and assigning a depth to those cuttings are routine procedures during drilling operations.
The pyrolysis of reservoir rock and/or rock cuttings has been employed to determine the API gravity of oil and the composition of reservoir rock extracts. The pyrolytic method involves the heating of the sample in an inert atmosphere at an initial temperature of about 180.degree. C. When the sample is inserted in the heated chamber, the light volatile hydrocarbons are removed and analyzed. The temperature is subsequently increased and heavier free oil is thermovaporized. Above approximately 400.degree. C., hydrocarbons that have not been vaporized are thermally "cracked" to lighter hydrocarbons which are vaporized. The sample is heated to a maximum temperature of 600.degree. C. in the inert atmosphere. The hydrocarbons released during these heating stages are quantified, as by a flame ionization detector ("FID"). If a complete analysis is required, the sample is contacted with a stream of oxygen or air at about 600.degree. C. and the resulting CO.sub.2 is analyzed by a thermal conduction detector ("TCD".)
Data plots of hydrocarbons released as a function of temperature can be produced on commercially available equipment. One such pyrolysis device and related analytical equipment is commercially available from the Institut Francais du Petrole through its distributor Vinci Technologies, (both of Rueil-Malmaison, France) under the trademark ROCK-EVAL. Another supplier of pyrolytic instrumentation is Humble Instruments & Services, Inc., of Humble, Tex.
As used in this specification and claims, the following terms have the meanings indicated:
HC means hydrocarbons.
ln means natural logarithm.
LV is the weight in milligrams of HC released per gram of rock at the static temperature condition of 180.degree. C. (when the crucible is inserted into the pyrolytic chamber) prior to the temperature-programmed pyrolysis of the sample.
TD is the weight in milligrams of HC released per gram of rock at a temperature between 180.degree. C. and T.sub.min .degree. C.
TC is the weight in mg of HC released per gram of rock at a temperature between T.sub.min .degree. C. and 600.degree. C.
LV+TD+TC represents total HC vaporizing between 180.degree.-600.degree. C. A low total HC indicates rock of lower porosity or effective porosity. A low value can also indicate zones of water and/or gas.
POPI.sub.o is the value of the pyrolytic oil productivity index as calculated for a representative sample of crude oil of the type which is expected to be found in good quality reservoir rock in the region of the drilling and chosen as a standard.
T.sub.min (.degree.C.) is the temperature at which HC volatization is at a minimum between the temperature of maximum HC volatization for TD and TC and is empirically determined for each sample. Alternatively, a temperature of 400.degree. C. can be used for samples where there is no discernable minimum between TD and TC. The latter sample types generally have very low total HC yields.
Phi is the average porosity of the rock.
Sxo is the saturation of drilling mud filtrate and represents the amount of HC displaced by the filtrate, and therefore, movable HC.
Phi*Sxo vs depth plot--the area below the curve represents the proportion of porosity which contains movable HC.
Phi vs depth plot--the area between the Phi curve and the Phi*Sxo curve represents immovable HC, or tar.
Gamma--the naturally occurring gamma rays that are given off by various lithologies while measuring directly in the well bore by the prior art petrophysical tools and are reported in standard API (American Petroleum Institute) units.
Caliper--the measured diameter of the well bore taken at the time of running petrophysical logs.
Density porosity--the porosity calculated by prior art methods from the petrophysical bulk density tools using an assumed fluid and grain density.
Neutron porosity--the porosity measured by prior art methods from petrophysical neutron tools.
Deep resistivity--the resistivity measured by deep invasion (long spacing between source and receiver), lateral log or induction petrophysical tools which is used as a measurement of undisturbed formation resistivity.
Medium resistivity--the resistivity measured by medium invasion (medium spacing between source and receiver), lateral log or induction petrophysical tools which is used as a measurement of resistivity of the formation that has been flushed by mud filtrate from the drilling fluid.
Shallow resistivity--the resistivity measured by shallow invasion (short spacing between source and receiver), lateral log or induction petrophysical analytic techniques which is used as a measurement of the resistivity of the mud filtrate from the mud cake that forms on the interior of the well bore during drilling operations.
Neutron-density cross-plot porosity (N-D Phi)--the porosity determined from a common prior art method which compensates for the effects of lithologic and fluid changes that lead to inaccuracies in employing either density or neutron porosity measurements by themselves.
Core plug permeability--the permeability measured by prior art methods from cylindrical rock samples that are cut from cores taken from the drilling process that is reported in units of millidarcys (md).
In a typical pyrolytic data plot of oil-productive reservoir rock prepared in accordance with prior art methods, the first peak, which is detected when the sample is first placed in the pyrolysis oven at the initial temperature of 180.degree. C. and before the temperature program begins, is from the volatile components still present in the sample after sample preparation. These will be referred to as the Light Volatile Hydrocarbons, reported in milligram per gram rock sample, and represented by LV or LVHC. As the temperature program proceeds, a plot of temperature vs. released hydrocarbons detected results in a curve that first increases from the starting point at 180.degree. C., then gradually falls off to a minimum value in the vicinity of 400.degree. C..+-.20.degree. C. where thermocracking of the heavier petroleum components begins to occur. As thermocracking proceeds with increasing temperature, released hydrocarbons detected increase to a maximum and then fall off as the rock cutting sample reaches a maximum temperature of about 600.degree. C. For any given sample, the minimum temperature point between the two peaks is referred to as T.sub.min. The area under the first peak between 180.degree. C. (i.e., the starting point) and T.sub.min represents the total weight of hydrocarbons released in that temperature range, generally reported as milligrams per gram ("mg/g") of rock sample, and are referred to as the Thermally Distilled Hydrocarbons and represented as TD or TDHC. The area under the second peak between T.sub.min and 600.degree. C. represents the total weight of hydrocarbons that are first thermally cracked before thermal distillation from the substrate and detection and are reported in mg/g of rock sample, and are referred to as the Thermally Cracked Hydrocarbons (TC or TCHC). Various techniques for analyzing the pyrolysis data represented by LVHC, TDHC and TCHC have been practiced in the art.
In the pyrolytic analysis process, small samples (e.g., .ltoreq.100 mg) of powdered rock are placed in a steel crucible. The crucible is placed in a furnace and the sample is heated in a stream of helium gas to an initial temperature of 180.degree. C. After heating at 180.degree. C. for about three minutes, the temperature is increased. The rate of increase in the temperature is about 25.degree. C./min. or less, and preferably about 10.degree. C./min, and progresses from 180.degree. C. to about 600.degree. C.
The helium gas carries hydrocarbon products released from the rock sample in the furnace to a detector which is sensitive to organic compounds. During the process, three types of events occur:
1) Hydrocarbons that can be volatilized at or below 180.degree. C. are desorbed and detected while the temperature is held constant during the first 3 minutes of the procedure. These are called light volatile hydrocarbons (LVHC or LV). PA1 2) At temperatures between 180.degree. C. and about 400.degree. C., thermal desorption of solvent extractable bitumen, or the light oil fraction, occurs. These are called thermally distilled hydrocarbons or "distillables" (TDHC or TD). PA1 3) At temperatures above about 400.degree. C., pyrolysis (cracking) of heavier hydrocarbons, or asphaltenes, occurs. The materials that thermally crack are called thermally cracked hydrocarbons or "pyrolyzables" (TCHC or TC). PA1 (a) collecting the rock cuttings from a first location; PA1 (b) preparing the rock cuttings for pyrolytic analysis; PA1 (c) subjecting the prepared rock cuttings to pyrolytic analysis to provide data corresponding to LV, TD and TC; PA1 (d) graphically plotting the relationship expressed by the value of: PA1 (e) repeating said steps (a)-(d) above for rock cuttings obtained from a plurality of different locations displaced known distances from said first location to provide a graphic plot; and PA1 (f) identifying the vertical intervals on said graphic plot corresponding to POPI values as determined by formula (I) of: PA1 1) To 1 cc of the oil sample, add 9 cc of a suitable solvent, such as methylene chloride, dimethyl sulfide or other suitable solvent that will completely dissolve the oil sample and that is readily evaporated at 60.degree. C. Characteristics of solvents?! PA1 2) Prepare 9 steel crucibles with approximately 100 mg of clear silica gel. PA1 3) Apply to the silica gel, using an accurate syringe, three samples each of the solution of the oil in solvent in quantities of 10, 20, and 30 micro-liters. PA1 4) Dry the samples at 60.degree. C. in a vacuum oven for 4 hours. PA1 5) Subject the samples to pyrolytic analysis, using 100 milligrams as the required input sample size for the instrument, to provide data corresponding to LV, TD, and TC. PA1 6) Utilize standard spreadsheet and graphics software to input the data and prepare a plot with the y-parameter being the POPI value and the x-parameter being the sum of total hydrocarbons (LV+TD+TC). PA1 7) Select the range for the value of POPI.sub.o from the chart where the value of total hydrocarbons is between 4-6 milligrams per gram of sample. PA1 A POPI greater than about POPI.sub.o, indicates oil-producing reservoir rock; PA1 a POPI between 0 and 1/2POPI.sub.o indicates tar-occluded or non-reservoir rock; and PA1 a POPI between about 1/2 POPI.sub.o and POPI.sub.o indicates marginally oil-producing reservoir rock.
These events give rise to three `peaks` on the initial instrument output (referred to as a pyrogram). The peak for the static 180.degree. C. temperature is a standard output parameter of either the Vinci or Humble instruments. It is referred to as either S.sub.1 or volatile total petroleum hydrocarbons (VTPH), respectively. In the present invention, the value will be referred to as LV, as described above. Data generated from the temperature programmed pyrolysis portion of the procedure is reprocessed manually by the operator to determine the quantity of hydrocarbons in milligrams per gram of sample above and below T.sub.min. This reprocessing is a trivial exercise for an experienced operator and can be accomplished routinely with either the Vinci or Humble instruments. The first peak above 180.degree. C. represents the amount of thermally distillable hydrocarbons in the sample and is referred to as TD, the second peak above 180.degree. represents the amount of pyrolyzables or thermally "cracked" hydrocarbons in the sample and is referred to as TC. In the case of lighter hydrocarbons or the analysis of oil samples directly for calibration, T.sub.min may not be discernable. In this case, if the sample analysis is repeatable at 400.degree. C., the values of LV, TD, and TC employed in the method of the present invention are with respect to the specific temperature ranges defined above.
In other pyrolytic methods known to the prior art, measurement of released hydrocarbons was undertaken in the range up to 180.degree. C. and identified as S.sub.1, or volatile total petroleum hydrocarbons (vTPH) while S.sub.2 or pyrolyzable total petroleum hydrocarbon (pTPH) was the value associated with hydrocarbons released between 180.degree. C. and 600.degree. C.
The prior art methods for collecting and analyzing the data obtained by pyrolytic analysis have been found to be of limited value in making reliable determinations of the quality and condition of reservoir rock, particularly in regions of tar mats and occlusions.
It is often the case that tar mats are found between productive reservoir regions. Tar mats can be defined as high concentrations of bitumens enriched by asphaltenes. They form more or less continuous layers in the porous medium of the reservoir rock that can range from several feet to tens of feet in thickness and constitute barriers impermeable to the flow of crude oil.
Delays in obtaining information on the character and condition of reservoir rock can be especially costly when the drilling operation is being conducted "horizontally." As used hereafter in reference to well drilling operations, the term "horizontal" means wells bored outwardly from the nominally vertical well shaft or bore leading from the earth's surface. These horizontal wells are drilled for the purpose of exploring areas horizontally displaced from the vertical well shaft. Horizontal drilling is typically undertaken in an effort to increase the total footage of productive reservoir rock encountered by the well bore. Because of the potential for rapid changes in conditions from one area to another in the horizontal plane, it is desirable to characterize the reservoir rock as quickly as possible. Discontinuing drilling operations while awaiting analytical data can incur significant costs, and the costs of utilizing the MWD or LWD analytical techniques described above are also very high.
As will be apparent to one familiar with the costs involved, it would be particularly advantageous to be able to identify the presence of tar mats on something approaching a "real time" basis as the horizontal drilling operation proceeds. This information would permit the direction of the drill to be changed "on the fly" once the tar mat was detected.
It is therefore an object of this invention to provide an improved method, that is timely and cost efficient, for determining the quality and condition of reservoir rock during petroleum exploration drilling operations.
It is another object of the invention to provide a method for utilizing pyrolytic analysis data to differentiate between good and excellent quality reservoir rock.
It is also an object of the invention to provide an improved method of employing data from the pyrolytic analysis of rock cuttings for determining the character and quality of reservoir rock, including the existence of zones of low porosity rock and rock of low effective porosity.
It is a further object of the invention to provide a method from which information concerning the quality and condition of the reservoir rock can be quickly derived in the field and at the drilling site so that any changes in the direction of drilling can be made "on the fly" to maintain the position of the drill bit in the stratigraphic region of optimum production.
It is yet another object of the invention to provide a method by which the presence of tar mat in the vicinity of the drilling bit can be quickly and reliably determined by analysis of rock cuttings.
It is also an object of this invention to provide a reliable method for determining when the well bore has proceeded from oil-productive reservoir either structurally higher into a gas cap, if present, or downward below an oil-water contact.